“…To understand what we mean by BBM being borderline, it is useful to consider BBM as a special case of a class of Gaussian processes labelled by a function A : [0, 1] → [0, 1] with A(0) = 0, A(1) = 1 which is increasing and rightcontinuous. Given such a function, so-called variable speed branching Brownian motion [20,21,31,11,12] can then be constructed in two equivalent ways 1 .…”